Method of forming a film having enhanced reflow characteristics at low thermal budget

ABSTRACT

A method of forming a film having enhanced reflow characteristics at low thermal budget is disclosed, in which a surface layer of material is formed above a base layer of material, the surface layer having a lower melting point than the base layer. In this way, a composite film having two layers is created. After reflow, the surface layer can be removed using conventional methods.

TECHNICAL FIELD

The present invention relates to the fabrication of integrated circuitdevices and, more particularly, to a method of forming a film havingenhanced reflow characteristics at low thermal budget.

BACKGROUND

A primary goal in the semiconductor industry is high device yields.Manufacturers often must perform certain process steps on substantiallyplanar wafer surfaces to achieve this end. Where manufacturers attemptto perform these process steps on non-planar surfaces, various problemscan occur, resulting in a substantial number of inoperable devices and,therefore, low yield. For example, manufacturers must form metal lineson top of a substantially planar surface to ensure continuity over stepsat a reduced thermal budget.

Manufacturers frequently use a layer of oxide, such asborophosphosilicate glass ("BPSG"), to planarize the surface of waferscontaining advanced dynamic random access memory ("DRAM") structures,which have large stacked capacitor heights and high device density. Inone process flow, manufacturers deposit a film superjacent tohigh-profile device structures. The wafer is then heated to reflow thefilm. Finally, the wafer is made substantially planar by utilizing aconventional technique, such as chemical mechanical planarization("CMP").

Reflow is necessary to fill in voids that are created when the film isinitially deposited and to smooth the top surface of the film. Forreflow to occur, the film must be heated initially to its melting point.Surface tension prevents the film from reflowing even at its meltingpoint. Therefore, the film must be heated even further to overcome theeffects of surface tension and, thus, reflow.

However, there are severe limitations on the maximum thermal budget thatcan be tolerated during the fabrication of state of the art integratedcircuit devices. Thermal process steps can cause unwanted diffusion ofdopants and destabilization of existing structures. Therefore,manufacturers must carefully restrict the times, temperatures andpressures associated with each thermal process step.

This is particularly true following the source-drain implantation in themanufacture of DRAMs. Reflow of film and activation of source-drainimplants are thermal process steps that contribute significantly to thetotal thermal budget for fabrication of DRAM devices.

For submicron device fabrication, manufacturers are increasingly usingrapid thermal processing ("RTP") steps at high temperatures to achieveshallower junctions and reduced diffusion of dopants. Even where RTPreflow steps at high temperature are employed, it is crucial to utilizefilms having good reflow properties. RTP steps provide process windowsof short duration. Films having good reflow properties permit themanufacturer to use narrow process windows for annealing the film toeliminate the effects of moisture, aging and water absorption, whichcould otherwise cause void formation and segregation.

For these reasons, manufacturers seek films having enhanced reflowproperties at low thermal budgets. The reflow properties of certainfilms at lower thermal budgets can be enhanced through the introductionof specific dopants, such as boron, phosphorous or germanium, into thematerial forming the film. However, these doped films often exhibitother undesirable film properties when the concentration of dopants isincreased. For example, high concentrations of germanium in BPSG filmmake the film unstable and moisture-sensitive. Also, the germanium candiffuse into a subjacent active region, thereby unintentionally alteringdevice performance.

SUMMARY OF THE INVENTION

The current invention provides a method of forming a film havingenhanced reflow properties at low thermal budget, but not exhibiting theabove-mentioned drawbacks.

According to the present invention, a surface layer of material isformed above a base layer of material. The materials are chosen suchthat the surface layer has a lower melting point than the base layer. Inthis way, a composite film having two layers is created. After reflow,the surface layer can be removed using a conventional technique, such asCMP.

The advantages of the composite film over conventional films arenumerous and include the following: (1) The surface tension of thecomposite film is lower than that of the base layer, if it were usedalone, resulting in enhanced reflow characteristics at low thermalbudget; (2) no change is required in the basic composition and chemistryof the constituent materials; (3) the etching characteristics of thebase layer are not altered; (4) the surface layer can be formed andsubsequently removed using conventional process steps; (5) formation andremoval of the surface layer can be easily integrated into aconventional process flow; (6) the base layer remainsmoisture-resistant; and (7) the base layer acts as a diffusion barrierbetween underlying active regions and any dopants in the surface layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-section of a semiconductor substrate onwhich several high-profile devices have been formed.

FIG. 2 is a schematic cross-section of the structure resulting from thedeposition of a base film superjacent to the substrate depicted in FIG.1.

FIG. 3 is a schematic cross-section of the structure resulting from theformation of a surface film superjacent to the base film depicted inFIG. 2.

FIG. 4 is a schematic cross-section of the structure resulting from thereflow process as performed on the structure depicted in FIG. 3.

FIG. 5 is a schematic cross-section of the planarized substrateresulting from the application of a conventional planarizing technique,such as chemical mechanical planarization, to the structure depicted inFIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, several high-profile integrated circuit deviceshave been formed on a substrate 10 to create a device profile 14.Although not to scale, device profile 14 illustrates the high aspectratios and sharp angles that are characteristic of non-planarsemiconductor fabrication technologies, such as stacked cell designs forDRAM arrays.

Referring to FIG. 2, a film, base layer 11, has been formed superjacentto device profile 14. Preferably, base layer 11 comprises BPSG and has athickness sufficient to permit subsequent planarization of deviceprofile 14. Generally, base layer 11 can comprise any of a number ofmaterials, including GeBPSG, BPSG, BSG, PSG, SiO2 and organic chemistryfilm. Further, base layer 11 can be formed by a variety of knowntechniques, such as plasma enhanced deposition, chemical vapordeposition, low pressure chemical vapor deposition, rapid thermalchemical vapor deposition and sputter deposition.

When base layer 11 is initially formed, it does not perfectly conform todevice profile 14. For example, voids 12 exist between base layer 11 anddevice profile 14. Such voids 12 are especially likely to occur atcorners 15 of device profile 14. Moreover, voids 12' exist within baselayer 11 itself. Voids 12 and 12' can cause field leakage, decreasereliability by exposing an underlying device structure during or after asubsequent etch step, and have other deleterious effects on deviceperformance and, therefore, yield. Also, upper surface 16 of base layer11 is not planar at this step in the process.

Referring to FIG. 3, a second film, surface layer 13, has been formedsuperjacent to base layer 11 to create a composite film 17. Preferably,surface layer 13 lies directly on top of base layer 11. In general,however, one or more intermediate layers may exist between base layer 11and surface layer 13. An intermediate layer might be introduced for avariety of reasons, such as to act as a diffusion barrier. Constituentlayers 11 and 13 are chosen such that the melting point of surface layer13 is lower than that of base layer 11. Preferably, surface layer 13comprises GeBPSG and has a thickness of at least 1000 angstroms.Generally, surface layer 13 can comprise any of a number of materials,including GeBPSG having a germanium concentration higher than that ofthe base layer, BPSG having a boron concentration higher than that ofthe base layer, BSG having a boron concentration higher than that of thebase layer, BPSG having a phosphorous concentration higher than that ofthe base layer, PSG having a phosphorous concentration higher than thatof the base layer and organic chemistry film having a melting pointlower than that of the base layer.

So formed, composite film 17 has film properties superior to that ofeach of its constituent layers 11 and 13, if they alone were used. Forexample, the surface tension of composite film 17 is lower than that ofbase layer 11 because surface layer 13 has a lower melting point thanbase layer 11. Accordingly, film 17 has improved reflow properties overlayer 11, if it alone were used. This result is accomplished withoutchanging the basic composition and chemistry of constituent layers 11and 13. Consequently, the etching characteristics of base layer 11 arenot altered. Moreover, surface layer 13 can be formed using any of avariety of conventional process steps described below. After reflow,surface layer 13 can be removed using a conventional process step, asdescribed below. Therefore, formation and removal of surface layer 13can be easily integrated into a conventional process flow.

Films having superior reflow properties are often moisture-sensitive. Bycombining a surface layer 13 having superior reflow properties with abase layer 11 that is moisture-resistant; the best characteristics ofboth films become manifest in composite film 17. Themoisture-sensitivity of surface layer 13 is not a concern because thelayer can be removed by a subsequent process step in a conventionalprocess flow.

Finally, films having superior reflow properties often contain dopantsthat will diffuse into subjacent active regions. By combining a surfacelayer 13 having superior reflow properties with a base layer 11 thatcontains no dopants or has a relatively low concentration of dopants,the best characteristics of both films become manifest in composite film17. Dopants in surface layer 13 are less of a concern because base layer11 acts as a diffusion barrier between subjacent active regions anddopants in surface layer 13. Moreover, surface layer 13 will likely beremoved by a subsequent planarization step in a conventional processflow.

Surface layer 13 can be formed in several alternative ways. Preferably,surface layer 13 is formed by implanting a dopant into the top surfaceof base layer 11. In this manner, a top portion of original base layer11 becomes surface layer 13. The reflow properties of certain films atlower thermal budgets can be enhanced through the introduction ofspecific dopants. By implanting such a dopant into a top portion oforiginal base layer 11, the melting point of surface layer 13 isdecreased relative to base layer 11. Preferably, the dopant is germaniumand surface layer 13 has a thickness of at least 1000 angstroms.However, any one or more of a number of dopants, including boron,phosphorous and germanium could be used. A source of germanium fordoping layer 11 would be GeO₂.

Suitable implantation processes are known in the art. Generally, whenlayer 13 is formed by implantation, the thickness of surface layer 13can be controlled through the energy of the implant, with higherenergies resulting in a thicker surface layer 13. Further, the meltingpoint and, therefore, surface tension of surface layer 13 can becontrolled through the dose of the implant, with higher dosescorresponding to lower melting points. Preferably, the energy isapproximately 100 keV and the dose is approximately 1×10¹⁴ to 1×10¹⁶atoms/cm³.

Alternatively, surface layer 13 could be formed by exposing base layer11 to a dopant-containing gas, causing the dopant to diffuse into a topportion of base layer 11. For example, a dopant could be introduced intobase layer 11 through plasma immersion ion implantation, a process knownin the art. Again, the dopant could be any one or more of a number ofdopants, including germanium, boron and phosphorous. Dopant-containinggases suitable for this purpose include GeH4 or GeF4 as a germaniumsource, B2H6 as a boron source and PH3 as a phosphorous source. Oneadvantage of this alternative is that it permits merging the processstep of forming surface layer 13 with the process step of reflow.Specifically, the dopant-containing gas could be introduced into theprocessing chamber where reflow is occurring.

Alternatively, surface layer 13 could be formed by depositing anadditional layer of material above base layer 11, with or withoutintermediate layers of material. Generally, surface layer 13 cancomprise any of a number materials, including GeBPSG having a germaniumconcentration higher than that of the base layer, BPSG having a boronconcentration higher than that of the base layer, BSG having a boronconcentration higher than that of the base layer, BPSG having aphosphorous concentration higher than that of the base layer, PSG havinga phosphorous concentration higher than that of the base layer andorganic chemistry film. Further, surface layer 13 can be deposited by avariety of techniques known in the art, such as plasma enhanceddeposition, chemical vapor deposition, low pressure chemical vapordeposition, rapid thermal chemical vapor deposition and sputterdeposition.

Referring to FIG. 4, a schematic view of composite film 17 after reflowis illustrated. Voids 12 and 12', in FIG. 3, have been eliminated andupper surface 18 of surface layer 13 has been smoothed relative to itsstate prior to reflow. Surface layer 13 is thicker in low areas 19 ofupper surface 18 because the surface layer material tends to flowtowards low areas 19 during reflow. Reflow can be accomplished in anumber of ways that are known in the art, including RTP and conventionalfurnace processing. At this point in the process flow, surface layer 13has served its purpose of reducing the surface tension of composite film17 during reflow and can, therefore, be treated as a sacrificial film.

Referring to FIG. 5, wafer surface 19 is made substantially planarthrough the use of a conventional planarizing technique, such aschemical mechanical planarization. In this way, wafer surface 19 isreadied for subsequent processing steps. As an added benefit,sacrificial surface layer 13, in FIG. 4, is completely removed if baselayer 11 is of sufficient thickness. This planarization step wouldtypically be performed in a conventional process flow utilizing a priorart film. Consequently, the method of the present invention is easilyintegrated into a conventional process flow.

Although we have illustrated and described a present preferredembodiment of the invention and variations thereon, the invention is notlimited thereto but may be embodied otherwise within the scope of thefollowing claims.

We claim:
 1. A method of forming a film, comprising the steps of:forminga surface layer of material lying above a base layer of material, thesurface layer having a lower melting point than the base layer, whereinthe step of forming the surface layer comprises the step implanting atop portion of the base layer with a dopant; exposing said surface layerand said base layer to radiant energy so as to cause reflow of saidsurface layer and said base layer, removing said surface layersubsequent said reflow.
 2. The method of claim 1, wherein the dopantcomprises an element selected from the group consisting of germanium,boron and phosphorus.
 3. The method of claim 1, wherein the energy ofthe implant is approximately 100 keV and the dose of the implant isapproximately 1×10¹⁴ to 1×10¹⁶ atoms/cm³.
 4. The method of claim 1,wherein the step of forming the surface layer comprises the step ofdepositing the surface layer above the base layer.
 5. The method ofclaim 1, wherein the base layer comprises a material selected from thegroup consisting of GeBPSG, BPSG, BSG, PSG, and SiO2.
 6. A method offorming a film, comprising the steps of:forming a surface layer ofmaterial lying above a base layer of material, the surface layer havinga lower melting point than the base layer, wherein the step of formingthe surface layer comprises the step of exposing the base layer to a gascontaining a dopant; exposing said surface layer and said base layer toradiant energy so as to cause reflow of said surface layer and said baselayer, removing said surface layer subsequent said reflow.
 7. The methodof claim 6, wherein the dopant comprises an element selected from thegroup consisting of germanium, boron and phosphorus.
 8. The method ofclaim 7, wherein reflow is accomplished using rapid thermal processing.9. The method of claim 7, wherein reflow is accomplished usingconventional furnace processing.
 10. A method of forming a film,comprising the steps of:forming a surface layer of material lying abovea base layer of material, the surface layer having a lower melting pointthan the base layer, wherein the surface layer comprises a materialselected from the group consisting of GeBPSG having a germaniumconcentration higher than that of the base layer, BPSG having a boronconcentration higher than that of the base layer, BSG having a boronconcentration higher than that of the base layer, BPSG having aphosphorous concentration higher than that of the base layer, and PSGhaving a phosphorous concentration higher than that of the base layer;exposing said surface layer and said base layer to radiant energy so asto cause reflow of said surface layer and said base layer, removing saidsurface layer subsequent said reflow.